Pneumocystis carinii is a common cause of pneumonia in immunosuppressed patients, especially those with the acquired immunodeficiency syndrome. Techniques have been developed for detecting these organisms in bronchial lavage specimens with a high degree of reliability, often obviating the need for open lung biopsy, a far more traumatic procedure for the patient. However, meticulous attention to methodological detail is necessary to optimize detection of P. carinii in these specimens. This study has defined some of the more important variables for the detection of these organisms.